The present invention relates generally to improving the machining accuracy of a precision machine tool such as a lathe by testing the accuracy-affecting components of the tool to determine the most accurate operating range of the machine, placing the cutting tool in a rigid support device, and operating the tool under carefully controlled conditions.
The ultimate purpose of a machining technology is to produce parts which meet all specifications in the minimum possible time on the least expensive equipment, leading thereby to least cost. In the specific field of ultra precise machining wherein tolerances of the finished product are in the order of micrometers the difference of machine tool cost and cycle time between super precision tools and ultra precision tools is significant. The cost of the finished product is directly related to the machine tool cycle time, that is, the time required after machine set up to load, conduct the metal cutting operation and unload the machine. Machine tools referred to as xe2x80x9csuper precisexe2x80x9d tools typically provide precision in the order of plus or minus five micrometers but inherently operate significantly faster than machine tools referred to as xe2x80x9cultra precisexe2x80x9d tools which provide precision in the order of plus or minus one micrometer. Therefore cycle time can be reduced if super precise machine tools can be made to provide ultra precise results.
The dynamic load deformation behavior of the spindle of a machine tool such as a lathe depends on the distribution of the component mass and the damping characteristics of the entire spindle system. Super precision lathes are usually equipped with belt driven ball or roller bearing spindles and may have a collet closure mechanism. Typical run out values for this type of spindle are in the order of five micrometers. The major sources of spindle run out error are unbalanced spindle parts such as the chuck, collet closure mechanism, brake disk, and the like.
Cutting speed, feed rate, and depth of cut all have significant impacts on accuracy of machining. Each of these factors has its own operating range and limits. Changes in one parameter affect the other parameters. For example, to increase cutting speed requires consideration of spindle run out behavior, thermal resistance of the material of the cutting tool, tool chatter and the like. Feed rate will be limited by the tool nose geometry and tool deflection under cutting forces. Depth of cut will be limited by the tool cutting edge geometry, the stiffness of the cutting tool and the rigidity of the tool clamping device.
Hysteresis on the positioning of the tool slides on the machine results from back lash (slack) in the slide drive system. The small deflections of the slide drive system joints, from the position motor to the coupling to the ball drive screw to the slide to the tool, all build up under reaction to the cutting forces at the tool cutting surface. When the direction of the tool motion is changed, all these small deflections reverse, registering in the positioning system sensors but not actually moving the cutting tool. These registered changes without corresponding actual tool movement limit the precision of the machine tool.
The operating precision of super precise machine tools such as lathes can be significantly improved over their advertised design specifications by the combination of careful set up, testing of the specific machine tool to determine the most accurate operating parameters of accuracy-critical components, and controlling the operating of the machine tool to operate within the most accurate operating parameters. Set up includes minimizing physical looseness; verifying alignment and balance of rotating components, including the work piece to be machined; and securely mounting cutting tools in rigid tool holders. Testing includes determining optimal operating speeds to achieve the best precision, resonant frequencies to be avoided, and cutting tool feed rates that deliver the most precision. Controlling the tool operation includes selecting the most beneficial rotational speed, feeding the cutting tool into the work piece at the most accurate speed and depth of cut, and always feeding the cutting tool into the work piece in one direction.